Nickel powder for multilayer ceramic capacitors

ABSTRACT

A nickel powder for multilayer ceramic capacitors according to the present invention is characterized in that an average particle size of a nickel powder is 0.1 to 1.0 μm, and the number rate of a nickel powder having a particle size of 2 μm or more is not more than 700 per million. As a process for producing such nickel powder, a process in which slurry containing a nickel powder having an average particle size of 0.1 to 1.0 μm in the amount of 5 to 25% by weight is classified using a hydrocyclone in which powders in a powder-liquid mixture are classified into at least coarse particles and fine particles, is preferred. According to the nickel powder of the present invention, the content of coarse particles is low, the particle size distribution is narrow, and the surface roughness in the paste state is decreased, and therefore, the nickel powder is extremely suitable for multilayer ceramic capacitors.

TECHNICAL FIELD

The present invention relates to a nickel powder for multilayer ceramiccapacitors suitable as a material of conductive paste fillers forforming internal electrodes in multilayer ceramic capacitors.

BACKGROUND ART

Conductive metal powders such as nickel, copper, silver, etc., areuseful for forming internal electrodes in multilayer ceramic capacitors,and in particular, nickel powder has attracted attention since it isless expensive than conventional metal powders such as palladium powder,etc. Additionally, internal electrodes tend to be formed in thin layershaving a thickness of 1 to 2 μm in accordance with miniaturization andcapacity increase of capacitors, and therefore, it is required thatnickel powder have a particle size of not more than 1 μm.

As nickel powder which satisfies such properties, fine spherical nickelpowder having a purity of 99.5% or more by weight and a particle size of0.05 to 1.0 μm is disclosed in Japanese Unexamined Patent PublicationNo. Hei 3-280304. According to this nickel powder, since filling densityin the electrode layer can be increased, specific resistance of theelectrode layer after sintering is small and delamination (peeling) andcracking hardly occur.

In addition, as a process for production of nickel powder, a vapor phasereduction in which nickel chloride gas is reduced by hydrogen gas isgenerally used, recently. This method has an advantage in that sphericalnickel powders can be efficiently produced; however, nickel powdershaving a particle size of 1 μm or more are often included in theproduced nickel powders. The nickel powder having a particle size of 1μm or more easily causes problems such as shorting between electrodes orvoids in the electrode layer, and it does not have desirable properties.Therefore, high-quality nickel powder having no coarse particles of 1 μmor more is desired. Since nickel powder particles of 1 to 2 μm or 5 μm,depending on the conditions, are included even if an average particlesize thereof is 0.4 μm, a classification technique in which coarseparticles of 1 μm or more can be efficiently removed is desired.

The present invention has been made in consideration of the abovecircumstances, and objects thereof are as follows.

{circle around (1)} A nickel powder for multilayer ceramic capacitors isprovided in which the content of coarse particles is low and thedistribution of particle size is narrow.

{circle around (2)} A nickel powder for multilayer ceramic capacitors isprovided in which the surface roughness in the paste state is small.

DISCLOSURE OF INVENTION

The present inventors reduced nickel chloride by a vapor phasereduction, then prepared nickel powders in the paste state with respectto each nickel powder in which the contents of nickel powders having aparticle size of 2 μm or more differ, and examined the surface roughnessthereof. As a result, it was found that satisfactory surface roughnesswas not obtained in the case in which nickel powders having a particlesize of 2 μm or more exist at a rate of about 1400 per million; however,superior surface roughness was obtained in the case in which the nickelpowders exist at a rate of about 50 per million. It was inferred fromthis fact that the surface roughness in which there is no problem inpractical use would be obtained if nickel powder were classified untilnickel powders having a particle size of 2 μm or more exist at a rate of700 per million, which is about the intermediate value between 50 permillion and 1400 per million, and the present invention therefore wasattained. That is, a nickel powder for multilayer ceramic capacitorsaccording to the present invention is characterized in that an averageparticle size is 0.1 to 1.0 μm, and the nickel powders having a particlesize of 2 μm or more exist at a rate of not more than 700 per million.

Next, the present invention is more specifically explained.

A. Nickel Powder Characteristics

The nickel powder of the present invention is characterized in that anaverage particle size is 0.1 to 1.0 μm, and the number rate of thenickel powders having a particle size of 2 μm or more is not more than700 per million, as described above. When such nickel powder is used forthe paste, the uniformity of thickness of the internal electrode layerin ceramic capacitors is superior, and shorting between internalelectrodes is thereby improved. The content (number rate) of nickelpowder having a particle size of 2 μm or more can be obtained by takingand image-analyzing electron microphotographs of nickel powder, andcounting the total number of particles and particles having a particlesize of 2 μm or more.

The number rate of the nickel powders having a particle size of 2 μm ormore must be not more than 700 per million, and it is preferable thatthe number rate is not more than 300 per million, more preferably thatthe number rate is not more than 100 per million, and most preferablythat the number rate is not more than 50 per million.

It is preferable that the average particle size of the nickel powder beeven smaller; however, the nickel powders easily aggregate as they aremade finer, and consequently, voids are easily formed in internalelectrodes. Therefore, as nickel powder for forming an internalelectrode of about 1 μm, a particle having an average particle size of0.2 to 0.4 μm is desirable.

With respect to this nickel powder particle having an average particlesize of 0.2 to 0.4 μm, the content (number rate) of the nickel powdershaving a particle size of 2 μm or more is preferably not more than 50per million, and more preferably that the number rate is not more than20 per million. Furthermore, the number rate of the nickel powdershaving a particle size of 1 μm or more is preferably not more than 100per million, and more preferably that the number rate is not more than50 per million.

B. Preparation of Nickel Powder

The above nickel powder of the present invention can be prepared byvarious methods, and in particular, the vapor phase reduction in whichnickel chloride is reduced by hydrogen, etc., is a preferable methodfrom the viewpoint of control of particle size. Specifically, thefollowing processes may be mentioned.

{circle around (1)} Nickel chloride solid as a starting material isvaporized by heating and reacts with hydrogen gas (reduction process),and nickel powder is therefore obtained.

{circle around (2)} Nickel solid as a starting material is chlorinatedby contacting chlorine gas (chlorination process), nickel chloride gasis thereby produced and reacts with hydrogen gas (reduction process),and nickel powder is therefore obtained.

In the process {circle around (2)} of the above processes, the partialpressure of nickel chloride gas can be controlled in the reductionprocess by controlling the feed rate of chlorine gas in the chlorinationprocess or by mixing nickel chloride gas produced in the chlorinationprocess with inert gas. Thus, by controlling the partial pressure ofnickel chloride gas, the particle size of forming nickel powder can becontrolled, and as a result, the particle size of nickel powder can bestabilized and can be optionally controlled.

Incidentally, the reduction process in the above vapor phase reductionis carried out at a high temperature of about 1000° C. or more. Thenickel powder just after production is easily aggregated since it has ahigh temperature, and it is therefore desirable that it cool rapidly.Specifically, a process in which the produced nickel powder is forced tocool by inert gas such as nitrogen gas, etc., may be mentioned. As acooling process, a cooling device, etc., can be provided and be used inaddition to the reduction reaction system, and in particular, it isdesirable that inert gas for cooling be directly contacted with nickelpowder just after production from the viewpoint of suppression of theaggregation of nickel powder. In such a cooling process, the coolingrate in which the nickel powder just after production is forced to coolis preferably 30° C./second or more, more preferably 40° C./second ormore, and most preferably 50 to 200° C./second, and in addition, thetemperature to which it is to be cooled, from a temperature level of thereduction reaction, is preferably at least not more than 800° C., morepreferably not more than 600° C., and most preferably not more than 400°C. In addition, after such cooling, it is also preferable to furthercool the nickel powder to a temperature which is lower than the abovetemperature (for example, a temperature ranging from room temperature toabout 150° C.) at the same cooling rate.

Thus, a nickel powder of the present invention in which few coarseparticles having a particle size of 2 μm or more are included can beobtained by controlling reaction conditions and cooling conditions forproducing nickel powder.

In order to obtain the nickel powder of the present invention, it ispreferable that the above produced nickel powder be further classifiedby mechanical means and that the coarse particles be removed. In thefollowing, the specific classification process is explained.

C. Classification Process for Nickel Powder

As a process for classifying nickel powder having a particle size of 2μm or more, that is, coarse particles, general classifiers such ashydrocyclones, air classifiers, etc., may be employed. However, sincehydrogen chloride gas or nickel chloride is adhered or adsorbed at thesurface of the produced nickel powder and must usually be removed bywashing, a process in which the nickel powder is dispersed in water andis prepared in the slurry form and is then classified by removing coarseparticles using a hydrocyclone, is desirable.

D. Hydrocyclone

As a hydrocyclone, a two-liquid separating type in which fine particlesare discharged from the top portion of the apparatus and coarseparticles are discharged from the bottom portion thereof, and athree-liquid separating type in which super-fine particles, which arefiner than fine particles, are discharged from the top portion of theapparatus, fine particles are discharged from the middle portionthereof, and coarse particles are discharged from the bottom portionthereof, can be mentioned. Of these, the latter type is more preferablyused from the viewpoint of the particle size being delicatelycontrolled. The nickel powder discharged from the top portion and/or themiddle portion is defined as a nickel powder of the present invention.

It is preferable that material for the hydrocyclone be ceramic in orderto ensure corrosion resistance and wear resistance. As a ceramic,alumina or silicon nitride is desirable. The hydrocyclone may beoperated alone or in combination which two or more hydrocyclones inparallel, and mass production can be realized by parallel operation oftwo or more hydrocyclones, and the productivity can thereby beeffectively improved.

E. Slurry

As a medium when the nickel powder is prepared in the slurry form, watercan be preferably used as described above. That is, the slurry isobtained by dispersing nickel powder having an average particle size of0.1 to 1.0 μm into water and is fed into the hydrocyclone. The contentof nickel powder in the slurry (slurry concentration) is preferably 5 to25% by weight, more preferably 7 to 20% by weight, and most preferably 8to 15% by weight.

In the case in which the above three-liquid separating type ofhydrocyclone is used, it is not desirable that the slurry concentrationof nickel powder be less than 5% by weight since coarse particles wouldbe easily included in the slurry discharged from the top portion and/orthe middle portion. Furthermore, as the slurry concentration isincreased, the content of coarse particles in the slurry discharged fromthe top portion and the middle portion is rapidly decreased; however,when it is more than 20% by weight, the classification efficiencydecreases and many coarse particles are included in nickel powder. Whenthe content of the nickel powder in the slurry is within the aboverange, in particular, in the case in which it is 7 to 20% by weight andis preferably 8 to 15% by weight, it is desirable because themanufacturing efficiency and the classification efficiency areremarkably superior. By classifying nickel powder under such conditions,a nickel powder in which the number rate of coarse particles having aparticle size of 2 μm or more is not more than 50 particles per millionparticles and is extremely low, can be obtained.

F. Discharge Rate of Nickel Powder from Hydrocyclone

With respect to the discharge rate of nickel powder from the abovehydrocyclone, a discharge rate in which 5 to 80% of nickel powder inslurry fed into the hydrocyclone is discharged from a nozzle fordischarging fine particles of the hydrocyclone and the remainder isdischarged from a nozzle for discharging coarse particles, ispreferable. In this case, the hydrocyclone is a two-liquid separatingtype, and the nozzle for discharging fine particles means the above topportion and the nozzle for discharging coarse particles means the bottomportion.

As a more preferable discharge rate, the nickel powder discharged fromthe top portion (nozzle for discharging super fine particles) and/or themiddle portion (nozzle for discharging fine particles) of thehydrocyclone, using the above three-liquid separating type ofhydrocyclone, is preferably 5 to 80% by weight and more preferably 5 to70% by weight. In this case, it is the most desirable that the nickelpowder further discharged from the middle portion be 20 to 75% byweight. In the case in which coarse particles must be classified moreprecisely, it is desirable that the nickel powder discharged from themiddle portion be prepared in the slurry form according to the presentinvention and be fed into the hydrocyclone.

Thus, the nickel powder in the slurry discharged from the top portionand/or the middle portion of the hydrocyclone is separated from water bydecantation, filtration, etc., then some processes such as drying, etc.,are carried out, and a nickel powder of the present invention istherefore obtained.

The nickel powder is added to organic solvent such as terpineol, decanoletc., and cellulose-type of organic resin such as ethyl cellulose, etc.,and is mixed, plasticizers such as phthalic ester etc., are furtheradded therein, and a conductive paste is thereby prepared, and inaddition, an internal electrode in a multilayer ceramic capacitor isformed by the conductive paste. A nickel powder of the present inventioncan prevent failures such as shorting, delamination, etc., due to theroughness of the surface of the electrode, when it is used as aninternal electrode in a multilayer ceramic capacitor, because coarseparticles having a particle size of 2 μm or more are very rare.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a chart showing a relationship between slurry concentrationand discharge rate of nickel powder from “the top portion and the middleportion” according to an Example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, effects of the present invention will be furtherclarified by explaining Examples of the present invention.

EXAMPLE 1 Classification Test of Nickel Powder

Two kinds of nickel powder (average particle sizes of 0.62 μm and 0.65μm) were produced by a vapor phase reduction of nickel chloride and wereused as Comparative Example samples. The number rates of nickel powderhaving a particle size of 2 μm or more which existed in nickel powder ofthese samples (Nos. 1 and 2) were examined using the microscope. As aresult, the number rate in Sample No. 1 was 1405 per million and thenumber rate in Sample No. 2 was 736 per million.

Next, the nickel powders of each Comparative Example as described abovewere dispersed in water, respectively, and were prepared in the slurryform at a slurry concentration (percentage by weight of nickel powder inslurry) of 10% by weight. These slurries were fed into a three-liquidseparating type of hydrocyclone (trade name: TR-5 type super clone;produced by Murata Industry Co., Ltd.) at a feed pressure of 6 kg/cm²and were classified, and nickel powders were discharged from the topportion which was a nozzle for discharging fine particles and wereobtained as the nickel powder of the Example, respectively. The numberrates of nickel powder having a particle size of 2 μm or more whichexisted in nickel powder of these Examples were examined using themicroscope. As a result, the number rate in Sample No. 1 was 33 permillion and the number rate in Sample No. 2 was 42 per million. Theresults of the classification test are shown in Table 1.

TABLE 1 Comparative Example Example (before classification) (afterclassification) Sample No. 1 1405/million 33/million Sample No. 2 736/million 42/million

As is apparent from in Table 1, in both samples Nos. 1 and 2, the numberof nickel powder having a particle size of 2 μm or more in nickel powderclassified by the hydrocyclone remarkably decreased and nickel powderaccording to the present invention could be obtained.

EXAMPLE 2 Surface Roughness Test of Electrode Layer

With respect to the nickel powder of the above sample No. 1, averageparticle sizes thereof before classification and after classificationwere examined. Then, these nickel powders were prepared in paste formand were applied on a substrate; a liquid component was evaporated byheating the substrate; and an electrode layer was therefore formed, andthe surface roughness thereof was measured. Measurement conditions ofthe surface roughness, etc., are shown in the following.

{circle around (1)} paste composition:

α-terpineol (42% by weight) Ethyl cellulose  (3% by weight) Nickelpowder (55% by weight)

{circle around (2)} substrate: Glass

{circle around (3)} paste coater:

Screen printing machine

{circle around (4)} paste evaporation conditions:

Nitrogen gas atmosphere at 400° C.

{circle around (5)} surface roughness measuring apparatus:

Electron beam three-dimensional roughness analyzer

(trade name: ERA-8000; produced by Elionics Co., Ltd.)

{circle around (6)} surface roughness measurement:

The areas of portions surrounded by roughness curves measured on everyscanning line and center lines thereof were divided by the laterallength, and then average roughness of the center line was subtractedtherefrom. The measured value was defined as the surface roughness of 1visual field. In addition, an average of 10 visual fields was defined asthe surface roughness of the paste.

The measurement results of average particle size and surface roughnessare shown in Table 2.

TABLE 2 Comparative Example Example (before classification) (afterclassification) Average Particle Size (μm) 0.62 0.41 Surface Roughness(μm) 0.249 0.103

As is apparent from Table 2, the surface roughness of the electrodelayer in which nickel powder was prepared in paste form afterclassification was smoothed to half of the surface roughness thereofbefore classification, and it was confirmed to be very useful as anickel powder for multilayer ceramic capacitors.

EXAMPLE 3 Classification Test of Nickel Powder

The nickel powder having an average particle size of 0.4 μm (BETdiameter based on the specific surface conversion) produced by a vaporphase reduction of nickel chloride was dispersed in water and wasprepared in the slurry form, then it was fed in the three-liquidseparating type of hydrocyclone (trade name: TR-5 type super clone;produced by the Murata Industry Co., Ltd.), and the classificationefficiency due to differences between slurry concentrations (percentageby weight of nickel powder in slurry) was examined. Slurryconcentration, feed rate and feed pressure of slurry to hydrocyclone,and feed rate of nickel powder are shown in Table 3.

TABLE 3 Slurry Slurry Slurry Ni Powder Concentration Feed Rate FeedPressure Feed Rate (Ni Powder: wt %) (cc/min) (kg/cm²) (g/min) Test No.1 22.4 3775 6 846 Test No. 2 14.7 3686 6 542 Test No. 3 13.6 4083 6 556Test No. 4 11.0 3513 6 385 Test No. 5 7.1 3471 6 247 Test No. 6 5.3 34386 182 Test No. 7 4.4 3286 6 143

The discharge amounts of the nickel powder in the slurries dischargedfrom the top, middle, and bottom portions of hydrocyclone are shown inTable 4.

TABLE 4 Top and Middle Top Portion Middle Portion Portions BottomPortion (g/min (%)) (g/min (%)) (g/min (%)) (g/min (%)) Test No. 1 50.8(6) 587 (69) 637.8 (75) 208 (25) Test No. 2 17.7 (3) 365 (67) 382.7 (70)159 (30) Test No. 3 23.1 (4) 351 (63) 374.1 (67) 182 (33) Test No. 4 7.7 (2) 213 (55) 220.7 (57) 164 (43) Test No. 5  46.4 (19) 164 (66)210.4 (85) 36.1 (15)  Test No. 6  27.6 (15) 114 (63) 141.6 (78) 40.0(22)  Test No. 7  6.9 (5) 65.4 (46)   72.3 (51) 70.9 (49) 

A relationship between slurry concentration and discharge rate of nickelpowder of “the top portion and the middle portion” is shown in FIG. 1.As shown in FIG. 1, it was found that as the concentration of slurry fedto the hydrocyclone increases, the discharge amount of the nickel powderfrom the top and bottom portions increases; however, the dischargeamount of nickel powder drops once when the concentration is more thanabout 7% by weight, and the discharge amount gradually increases againwhen it is more than about 10% by weight.

Furthermore, the existence of nickel powders having a particle sizeabove 1 μm was examined by observing each sample using a microscope, andthe quality thereof was evaluated. The evaluated results are shown inTable 5. In addition, with respect to nickel powders discharged from thetop and middle portions, the contents (number rate) of nickel powderhaving particle sizes of 1 μm or more and 2 μm or more are examined,respectively. The results are shown in Table 6.

As is apparent from Tables 5 and 6, nickel powders which satisfy thequalities could be obtained from the top and middle portions at slurryconcentrations of Tests Nos. 2 to 5.

TABLE 5 Top Portion Middle Portion Bottom Portion Test No. 1 Δ × × TestNo. 2 ◯ Δ × Test No. 3 ◯ Δ × Test No. 4 ◯ Δ × Test No. 5 Δ Δ × Test No.6 Δ × × Test No. 7 Δ × × ◯: There is no particle having a particle sizeof 1 μm or more. Δ: There are a few particles having a particle size of1 μm or more, but there is no problem in practical use. ×: There is manyparticles having a particle size of 1 μm or more.

TABLE 6 Particles Having Particles Having A Particle Size of A ParticleSize of 1 μm or More 2 μm or More Average Particle Size (number (number(μm) rate per million) rate per million) Top Middle Top Middle TopMiddle Portion Portion Portion Portion Portion Portion Test No. 1 0.230.36 24 256 13 135 Test No. 2 0.22 0.35  0  55  0  36 Test No. 3 0.220.35  0  67  0  47 Test No. 4 0.20 0.31  0  82  0  51 Test No. 5 0.240.38 25 513 12 275 Test No. 6 0.23 0.35 76 745 51 328 Test No. 7 0.250.37 89 1570  58 628

The higher the concentration of the slurry fed to the hydrocyclone, thebetter the productivity, and the greater the weight of the nickel powderdischarged from the top and middle portions, the higher the yield of theclassified nickel powder. Therefore, the nickel powder of Test No. 2 isparticularly desirable in the above Examples.

What is claimed is:
 1. A nickel powder for multilayer ceramiccapacitors, wherein an average particle size thereof is 0.1 to 1.0 μm,and the number rate of nickel powder having a particle size of 2 μm ormore is not more than 700 per million.
 2. A nickel powder for multilayerceramic capacitors as recited in claim 1, wherein said number rate isnot more than 50 per million.
 3. A nickel powder for multilayer ceramiccapacitors as recited in claim 1 or 2, wherein said nickel powder isprepared by a vapor phase reduction.
 4. A nickel powder for multilayerceramic capacitors as recited in claim 3, wherein said vapor phasereduction is conducted by a method in which nickel chloride solid isused as a starting material and vaporized by heating, and vaporizednickel chloride gas is reacted with hydrogen gas.
 5. A nickel powder formultilayer ceramic capacitors as recited in claim 3, wherein said vaporphase reduction is conducted by a method in which nickel metal is usedas a starting material and is chlorinated by contacting chlorine gas,and produced nickel chloride gas is reacted with hydrogen gas.
 6. Anickel powder for multilayer ceramic capacitors as recited in claim 1 or2, wherein said nickel powder is classified by a hydrocyclone and saidhydrocyclone classifies powder in a powder-liquid mixture into at leastcoarse particles and fine particles.
 7. A nickel powder for multilayerceramic capacitors as recited in claim 6, wherein said nickel powder isobtained by feeding the slurry, in which nickel powder having an averageparticle size of 0.1 to 1.0 μm is contained in the amount of 5 to 25% byweight, into said hydrocyclone.
 8. A nickel powder for multilayerceramic capacitors as recited in claim 6, wherein said hydrocyclonecomprises a nozzle for discharging fine particles and a nozzle fordischarging coarse particles, and 5 to 80% of said nickel powder in theslurry is discharged from said nozzle for discharging fine particles andthe remainder is discharged from said nozzle for discharging coarseparticles in said hydrocyclone.
 9. A nickel powder for multilayerceramic capacitors as recited in claim 6, wherein said hydrocyclonecomprises a nozzle for discharging super fine particles, a nozzle fordischarging fine particles, and a nozzle for discharging coarseparticles, and 5 to 80% of said nickel powder in the slurry isdischarged from said nozzle for discharging super fine particles and/orsaid nozzle for discharging fine particles and the remainder isdischarged from said nozzle for discharging coarse particles in saidhydrocyclone.
 10. A nickel powder for multilayer ceramic capacitors asrecited in claim 9, wherein the amount of nickel powder from said nozzlefor discharging fine particles is 20 to 75% by weight.